1. Technical Field of the Invention
The present invention relates generally to a failure monitor for a motor drive control system which controls rotation of a motor working to output torque to a motor-driven member through a torque transmitting mechanism and an output shaft joined to the motor-driven member.
2. Background Art
In recent years, in order to meet space saving requirements, facilitating ease of assembly, or improving controllability of automotive vehicles, there have been increased trends toward use of an electrical system working to drive a controlled mechanism through an electric motor. For example, Japanese Patent First Publication No. 2002-323127 discloses an automatic transmission control system designed to actuate a range shift mechanism for automotive automatic transmissions using an electric motor. A selection of gear ranges of the automatic transmission is achieved by actuating the range shift mechanism using a drive shaft joined to an output shaft of the motor through a speed reducing mechanism. The motor has installed thereon an angular position sensor such as an encoder working to measure an angular position of the output shaft of the motor. The system uses an output of the angular position sensor to rotate the motor to bring the angular position thereof into agreement with a target one, thereby establishing a selected one of the gear ranges of the automatic transmission through the range shift mechanism.
The rotation of the motor is converted into that of the drive shaft (i.e., a manipulated variable of the range shift mechanism) through the speed reducing mechanism. A speed reducing mechanism of this type is typically made of a gear train in which there is inevitably some play or looseness between gears. In a case where the speed reducing mechanism is joined to the drive shaft through fitting of a D-shaped connector formed on the tip of an axis thereof into a mating recess formed in the drive shaft, some clearance is required to facilitate ease of such fitting, which will, however, result in an error in the amount by which the output shaft is rotated by the motor even if the motor is controlled accurately by monitoring the output of the angular position sensor as representing the angular position of the motor, thus leading to a difficulty in controlling the manipulated variable of the range shift mechanism correctly.
In order to compensate for the error in the amount by which the output shaft is rotated, an output shaft angular position sensor may also be used to measure the angular position of the output shaft for controlling the motor to bring the angular position of the output shaft into agreement with a target one under feedback control.
However, if an error in the output of the output shaft angular position sensor arises from some failure in operation thereof, it will result in an error in controlling the manipulated variable of the range shift mechanism. This may cause the automatic transmission to be shifted to an erroneous one of the gear ranges through the range shift mechanism and result in a difficulty in monitoring a malfunction of the feedback control system.